'72 Dart Swinger Rear-End Help

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nub340

nub340

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Hello,

My goal is a super street-able ride that can be driven daily if need be, with the ability to go out to the track or autocross once or twice a year. Right now my plan for the rear end is to upgrade the current Detroit Locker/3.73/Leaf/Drum/14" wheels to a TrueTrac/3.55/RMS 4-link/Disc Brakes/17x8 or 9" wheels.

I've already installed an Alterktion with 13" Wilwood SL6 brakes up front. However, I bought the rear-end ~15 years ago from a classified ad. I know it's an 8.75" with a 489 case but that's about it. After researching a bit I'm now realizing that there are many variants/sizes of this rear-end and I'm not 100% sure which one I have or if it matters if I want to install an RMS 4-link with 12" Wilwood disc brakes.

Specifically I am concerned that I might need to have the rear-end narrowed in order for the bigger wheels to fit properly, or if I can just compensate with some additional backspacing. When I installed the rear-end (~15 yrs ago) I noticed it was a bit wider than the stock 7.25", although it fit my existing leaf springs and stock wheels at the time. I suspect the rear end is not from an a-body and is wider than an a-body 8.75".

My understanding is that I'll also need to get my axles machined (or order new Moser axles) for the Wilwood brake kit to fit so I want to learn as much as possible before proceeding or ordering any parts.

Should I find out exactly which rear-end variant I have and have it narrowed or can I just compensate with wheel backspacing?? If I don't shorten the rear-end will it lead to any other issues or challenges later?

I would certainly appreciate any guidance if someone is familiar with this setup.

Thanks!!
 
I would determine exactly what rear you have. An A body rear is gonna be 52 5/8" flange to flange. 43" center to center on the spring perches.

If you only plan to run an 8 or 9 inch wheel, it should be a fairly straightforward deal.

But you ultimately need to form a plan in regards to tire size/width etc and plan from there.
 
Okay - so using the information at the links that @skep419 posted I was able to determine that my flange-to-flange width is 54 1/4" and the driver's side axle length is 28 7/8" (measured from outside axle flange to splined tip).

I believe this confirms my suspicion that it's a B-body 8.75" ('65-'67). If this is correct what course of action should I take considering my goals? Should I have the housing shortened to a-body specification and order some new Moser axle shafts to fit?
 
I wouldn't do anything until I decided what rims I wanted to use on the car and determined what offsets they were available in. Most of the 17 and 18" rims come in higher offsets, so the wider rear axle might be beneficial.

What kind of offsets are needed in the front? Matching the front and rear offsets would be best if you wanted to run the same size rims and tires all the way around so you could rotate tires.
 
I'm using that rear end on my 72 dart, custom backspacing for the wheels with about an 8" max wheel size (w/o moving the springs in). Also moved the perches in a half inch on each side.
 
Thanks guys - I know I'm a noob and some of this stuff might be common knowledge to y'all so I appreciate the help!!

@72bluNblu - with the RMS Alterktion you can go 5.5" of backspacing with 17" or larger wheels up front. I currently have a pair of 17x8" Torque Thrust IIs w/5.48" of backspacing up front and they fit very nicely with no rubbing. I was planning on running the same wheels in the rear, at least for a while.

@Lelo Dart - If I'm doing my math correctly, the difference in length between my rear-end and an a-body rear-end is:
54.25" - 52.625" = 1.625" (total)
1.625 / 2 = .8125" (per side)
If I were get another pair of 17x8" wheels, does that mean I would need to add an additional .8125" of backspacing to whatever is "normal" for an a-body rear-end in order for all corners to be square?
 
What rear do you currently have in the car. What is the bolt pattern, is it the same as the front, small or large??
 
Hey @Bad Sport - The rear is a B-body 8.75" (65-'67) with the large 4.5" bolt pattern, same bolt pattern as the front.
 
nub340 said:
Hey @Bad Sport - The rear is a B-body 8.75" (65-'67) with the large 4.5" bolt pattern, same bolt pattern as the front.
Click to expand...

So, if you already have it in the car, pull one of the wheels off and see how it fit's on the rear.
 
Yeah I threw them on just to see and they're a super tight fit. Enough to allow me to roll the car around in the garage/driveway but would definitely rub bad once I got on the road. Even the original 14" wheels with 215/70/14 tires rubbed pretty bad with the bigger rear, so I ended up installing air shocks to lift up the back a bit.

The new wheels have 245/45/17 tires and with the larger 5.48" backspace they pretty much fill up the entire width with only 1/2" space between the leaf spring and rim (and slightly less to the tire) and virtually no space between the sidewall and fender. With that said, this is with the old crappy air shocks still installed.

This time around I want to do things right and optimize for street handling, braking and all around drive-ability. I also want to stick with the Torque Thrust II wheels but their 8" wide wheel only goes up to 5.48" backspacing. If I want more I'd have to go to a 9.5" wheel which may or may not fit once I install the RMS 4-link..
 
nub340 said:
The new wheels have 245/45/17 tires and with the larger 5.48" backspace they pretty much fill up the entire width with only 1/2" space between the leaf spring and rim (and slightly less to the tire) and virtually no space between the sidewall and fender. With that said, this is with the old crappy air shocks still installed.
Click to expand...

I think you have your answer right there.

It sounds like a mini tub is in order. After that, set the wheel/tire combo that you "want" to run and what fits in there and measure for your rearend length. Once you have the rear and tires squared away so it fits nice and centered, then finish the 4 link install.

I'm not familiar with the how to's of installing a 4 link, other than what I have read. But the first order of business is getting the rear, wheel and tire combo correct.
 
nub340 said:
Yeah I threw them on just to see and they're a super tight fit. Enough to allow me to roll the car around in the garage/driveway but would definitely rub bad once I got on the road. Even the original 14" wheels with 215/70/14 tires rubbed pretty bad with the bigger rear, so I ended up installing air shocks to lift up the back a bit.

The new wheels have 245/45/17 tires and with the larger 5.48" backspace they pretty much fill up the entire width with only 1/2" space between the leaf spring and rim (and slightly less to the tire) and virtually no space between the sidewall and fender. With that said, this is with the old crappy air shocks still installed.

This time around I want to do things right and optimize for street handling, braking and all around drive-ability. I also want to stick with the Torque Thrust II wheels but their 8" wide wheel only goes up to 5.48" backspacing. If I want more I'd have to go to a 9.5" wheel which may or may not fit once I install the RMS 4-link..
Click to expand...

People have run 245's on stock cars without spring offsets without rubbing. At least allegedly. ;-) They may have modified the outer quarter lip though.

You can trim the quarter lips back almost a full 1/2" without getting into the spot welds that hold the outer wheel tub to the quarter lip. That's about all the room you need on the outside. On the inside, I run my tires about little less than 3/8" off my springs with my 295/35/18's (Duster, wider stock wheel tubs). I've never hit the springs. So, you may have a little room there.

Keep in mind that with the 4 link you'll have more room to the inside, even if you don't move the spring perches, because the 4 link bars are narrower than the springs. I'd bet you'd pick up an extra 1/2". Not only that, you could probably run the clearance smaller. The 4 link arms won't move around as much as the leaf springs, you'd only have to worry about tire deflection. I know that guys have run 275's on 9" wide rims on Darts with the 1/2" spring offset kits.

I would be looking for at least 6" of backspace on an 8" rim if that's how the 17x8's with 245's and 5.48" of backspace fit. Maybe even a little more, because you can always add a small spacer if you have a little too much backspace. Any spacer under a 1/4" is no big deal. So maybe shoot for a 6.25" backspace.

Also, most rear disk kits increase your track with about a 1/4" - 5/16". You'd want to add whatever the additional offset from the disks into the wheel specs. Not sure what the Wilwoods add, I seem to recall Dr. Diff's rear brake kit adding about 5/16" per side? Plus, his disk kit allows the use of tapered axle bearings, most of the other ones require green bearings. I try to use tapered axle bearings on all of my cars.
 
5.5 or 5.75 backspace with a 8" wheel if I remember right. If you know the backspace of the wheels you have then measure with them installed to find the correct BS to center them between the fender and spring.
 
Bad Sport said:
It sounds like a mini tub is in order. After that, set the wheel/tire combo that you "want" to run and what fits in there and measure for your rearend length. Once you have the rear and tires squared away so it fits nice and centered, then finish the 4 link install.
Click to expand...

Yep, again I want to run these AR 17x8 Torque Thrust II wheels but they only make them in so many sizes and the largest backspacing they offer (for 17x8" wheels) is 5.48". Which is really almost enough! If they had 5.75" of backspace I think I'd be in good shape.

72bluNblu said:
You can trim the quarter lips back almost a full 1/2" without getting into the spot welds that hold the outer wheel tub to the quarter lip. That's about all the room you need on the outside. On the inside, I run my tires about little less than 3/8" off my springs with my 295/35/18's (Duster, wider stock wheel tubs). I've never hit the springs. So, you may have a little room there.
Click to expand...

Yes I should definitely do this! This alone would immediately increase my sidewall to quarter panel clearance from virtually none to 1/2". Which may be just enough.

72bluNblu said:
Keep in mind that with the 4 link you'll have more room to the inside, even if you don't move the spring perches, because the 4 link bars are narrower than the springs. I'd bet you'd pick up an extra 1/2". Not only that, you could probably run the clearance smaller. The 4 link arms won't move around as much as the leaf springs, you'd only have to worry about tire deflection. I know that guys have run 275's on 9" wide rims on Darts with the 1/2" spring offset kits.
Click to expand...

This is interesting. Could I not then use a 1/2" spring offset kit to sort of "mock-up" the additional room I'll get when I install the 4-link??

72bluNblu said:
I would be looking for at least 6" of backspace on an 8" rim if that's how the 17x8's with 245's and 5.48" of backspace fit. Maybe even a little more, ...
Click to expand...

Yeah unfortunately AR doesn't make them in those sizes so I'm hoping I can figure out a solution to accommodate these ones.

72bluNblu said:
Also, most rear disk kits increase your track with about a 1/4" - 5/16". You'd want to add whatever the additional offset from the disks into the wheel specs.
Click to expand...

This didn't even occur to me, thanks for bringing it up! I'll def look into this further.

Thanks again for all the responses. :thumbup:
 
Lelo Dart said:
5.5 or 5.75 backspace with a 8" wheel if I remember right. If you know the backspace of the wheels you have then measure with them installed to find the correct BS to center them between the fender and spring.
Click to expand...

Yeah I would agree, 5.75" would have them sitting nice and center w/the leaf springs in their stock location.

Someone I know suggested that I have the wheels milled 1/8" but that doesn't sound like a good idea to me!
 
Okay so I trimmed off about half of the q-panel lip this weekend and I now have close to 1" of clearance with the torque thrust II wheels, 245/45/17 tires and stock drum brakes installed.

Which leads me to the brakes. Wilwood says their brake kits will add 1/4" per side, so this brings me back down to ~3/4" of clearance, which I'm thinking should be enough.

RMS offers two different rear brake kits and I'm looking at this one: https://www.reillymotorsports.com/store/product.php?productid=16192&cat=274&page=1

Which I think this is more or less this one:
http://www.wilwood.com/BrakeKits/BrakeKitsProdRear.aspx?itemno=140-10767-D

However the RMS website mentions that it requires custom-machined axles, but the Wilwood installation instructions do not mention this requirement. Does anyone know if I will really need to have my axles machined?
 
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